The present invention relates to a method and an apparatus for recording on optical disk media which are used for recording data processed by a computer or the like, recording audio or audio-visual signals, or recording and reproducing other information.
Along with the advancement of computers and of means for fast and massive transmission of information in recent invention, requirements have arisen for the development of nonvolatile memory apparatuses which are inexpensive but have high density and large capacity and excelling in capability to transfer information at high speed.
Apparatus having magnetic disk media as such nonvolatile memory units are generally used, but they in fact have the disadvantages of being inadequate in recording capacity and expensive per unit of information and, particularly, not permitting the interchange of recording media in a fixed type magnetic disk apparatus.
In this connection, technology for optically recording information on media is attracting keen interest as a solution to these problems and, especially, users in many different areas are pinning much hope on rewritable optical disk media and memory apparatuses using such media.
Usually, such an optical recording apparatus according to the prior art consists of a pulse current drive circuit, a bias current drive circuit for erasing use, a bias current drive circuit for reading use, an adder, and an optical head having a laser diode.
The pulse current drive circuit is so configured as to receive from outside original signals representing data to be recorded on an optical disk medium and to supply the adder with an output current according to the values of these original signals.
The bias current drive circuits for erasing use and that for reading use are so configured as to supply the adder with output currents when having respectively received switching signals.
In this prior art optical recording apparatus, a switching signal is supplied to the reading bias current drive circuit at the time of reading; the laser diode of the optical head is driven only by this output current of the reading bias current drive circuit; at the time of writing or erasing, a switching signal is supplied to the erasing bias current drive circuit; an output current corresponding to an original signal from the pulse current drive circuit and an output current from the erasing bias current drive circuit are supplied to the adder; and the laser diode of the optical is driven according to this added current value.
The pulse current drive circuit supplies a current by pulse drive as the potential of the original signal takes on a high level state or a low level state.
Moreover, the method of recording by this prior ark optical recording apparatus uses the laser irradiation power deriving from the Laser drive current at three separate steps including read power, erase power and write power, differentiated by the original signals and the switching signal.
By irradiating the recording layer of the optical disk medium with the laser beam, the medium temperature of the recording part is varied to the write temperature or the erase temperature to form pits corresponding to the pulse lengths of the original signals on the optical disk medium.
The optical recording apparatus and method according to the prior art, however, involves the following problem (see FIG. 5).
Thus, since the duration of irradiation with the laser beam is determined by the pulse length of the original signal, the medium temperature of a recording part corresponding to an original signal of a smaller pulse length does not become sufficiently high relative to that of a recording part corresponding to an original signal of a greater pulse length, resulting in an ultimate temperature difference on the recording layer of the optical disk medium between original signals of smaller pulse lengths and original signals of greater pulse lengths.
As a result, the time (.DELTA.t2) taken to vary the medium temperature at an original signal of a greater pulse length from the write temperature to the erase temperature becomes longer than the time (.DELTA.t1) taken to vary the medium temperature at an original signal of a smaller pulse length from the write temperature to the erase temperature, entailing a time lag.
Thus, the shapes of pits formed on the recording layer of the optical disk medium become uneven, leading to the problem of inviting deteriorations in carrier-to-noise ratio (CNR) and in jitter characteristic at the time of reading.
Furthermore, the use of the laser irradiation power deriving from the laser drive current at three separate steps including read power, erasing power and write power, differentiated by switching signals, leads to the problems that the procedure of adjustment an the time of setting the irradiation power of the laser beach from the optical head is made complex, and that the process required to adjust that irradiation power is increased.
Hereupon, it may be relevant to other examples of optical recording method according to the prior art and their problems.
According to examples disclosed in the Patent Disclosure Gazettes of No. 1989-43816 and No. 1989-59633, when information is to be recorded on an optical disk medium, the temperature of the recording part is controlled by varying the laser irradiation power.
However, since the laser irradiation power at the time of writing is varied when .information is to be recorded according to the methods of both these reference, there is the problem that multi-value laser irradiation power is required, entailing difficulty in temperature control besides the complex configuration of the current drive circuit for the laser diode.
On the other hand, according to examples disclosed in the Patent Disclosure Gazettes of No. 1988-214922 and No. 1990-239429, when information is to be recorded on an optical disk medium, the larger irradiation power is pulsized for the recording purpose.
However, by the method of the former reference, as temperature control is imprecise, there occurs the phenomenon that the temperature of the recording part varies with the width of the recording signal and the region of recording pit extension accordingly varies, making it impossible to accurately generate recording pits on the optical disk medium and to correctly recognize information on the pit edge.
On the other hand, according to the latter, although a long pulse is used for laser irradiation to achieve temperature control at the beginning of recording, there is the problem that the recording pit becomes greater than the signal to be actually recorded because the laser beam diameter and the extension of heat along with the temperature rise are not sufficiently taken into account. There is another problem that, unless the laser irradiation power is reduced, the magnitude of the signal to be recorded becomes unidentical with that of the recorded pit in which the signal is actually recorded, resulting in a deteriorated CNR. Furthermore, because of the lack of synchronism between the output of a signal generator for generating short pulses and the recording signal, the jitter characteristic is also deteriorated.